Ply-separability is desirable in a tissue product because a ply-separable web is believed by applicants to be more flexible than a conventional web of the same basis weight which is not ply-separable. The source of this hypothesis is the relation between thickness and flexural rigidity in a given material. The flexural rigidity of a member is proportional to the cube of its thickness in the direction in which a flexing force is applied. Thus, where a one-ply member has a thickness h, and thus a flexural rigidity proportional to h.sup.3, a two-ply member with a combined thickness of h (each ply is h/2 thick) has a flexural rigidity proportional to 2(h/2).sup.3, or h.sup.3 /4. Thus, in the case where the two plies of a two-ply member do not substantially interact at their interface, the flexural rigidity of the two-ply member is 1/4 that of a one-ply member of equal thickness. This illustrates that to reduce the bonding between well-bonded layers of a web, forming distinct plies which resemble plural noninteracting members, is to increase the flexibility of a paper web of a given thickness.
Laminate paper and paper-like structures exhibiting ply-separability are well known in the art. For example, U.S. Pat. Nos. 514,059, issued to Bird on Feb. 6, 1894; 1,606,428, issued to Kirschbraun of Nov. 9, 1926; 1,964,793, issued to Richter on July 3, 1934; and 2,234,457, issued to Strovink on Oct. 9, 1937 demonstrate that a nonadhesive material applied between two outer layers of paper or the like can be used to produce a ply-separable structure. However, none of this early work is believed by applicants to disclose the creation of a ply-separable web which is useful in the manufacture of absorbent paper products.
A United States Patent Application, Ser. No. 481,532 (series of 1970), filed on June 21, 1974 by Charles F. Dunning et al., entitled "Creped Laminar Tissue and Process of Manufacture" and serving as the priority document for German Offenlegungsschrift 25 28 311, laid open on Jan. 8, 1976, describes various methods for producing a three-layered ply-separable tissue web. The essence of the disclosure is that a web may be formed having outer layers of strongly bonded fibers separated by an intermediate zone wherein intralayer bonding is reduced. When the web is subjected to creping from a creping roll using a doctor blade, this reference teaches that only the layer of the tissue adjacent the creping roll is creped, and a second operation is believed to be necessary to crepe the other side of the web in order to create a finished product. This application also discloses that ply-separability is imparted to the web as a result of the creping operation which shear the two outer layers from each other. The result of this process is said to be a tissue product having two ply-separable layers, formed such that the crepe in one well-bonded layer is independent in both frequency and phase of the crepe in the other well-bonded layer. The applicants believe that this process would be much more expensive to produce soft webs than the process disclosed hereinafter, since an additional creping step is required to crepe both exterior surfaces of the web. Furthermore, the Dunning, et al. application specifically requires creping with a doctor blade as an essential element to produce a ply-separable product, while the applicants have found that unique aspects of the process disclosed hereinafter permit the use of many creping techniques.
The applicants are not aware of any prior art respecting the use of differential consistency in a paper web formed in two layers to create a ply-separable product.
U.S. Pat. No. 3,994,771, issued to Morgan et al. on Nov. 30, 1976 and commonly owned with this application, discloses the general type of papermaking machinery which may be used to produce paper according to the present invention. This reference, as well as others, can be distinguished from the present invention in that there is no teaching that this process can be used to make ply-separable products. Such a result is unexpected when using this process because, according to the teachings of Morgan et al, small isolated areas of the web strata are compressed together to form discrete densified areas corresponding to the knuckle pattern in the foraminous intermediate drying fabric, which would be expected to cause the two layers to adhere together rather than to separate. This reference does disclose, however, that the knuckle impression imparted by the drying fabric creates sites at which the web is predisposed to buckle, so that the mesh of the drying fabric influences the fineness of creping. This prior art reference discloses that a stratified web may be creped all the way through, although this teaching would not be expected to apply to the creping of a web to which ply-separability is imparted prior to creping.